Wiki info about Pz.III J1

Then next modification to the Pz.Kpfw.III medium tank after Ausf. J, featuring an enhanced armament.
These tanks, which were armed with KwK38 L/42 cannons, were unable to fight successfully against Soviet T-34 and KB tanks, whose armor could withstand the impact of projectiles fired by the short-barreled cannon. The tank’s armament had to be enhanced. The new weapon selected was the long-barreled KwK39 L/60, which featured high ballistic characteristics. However, despite these enhanced characteristics, fighting Soviet tanks remained extremely difficult. Tanks with the new armament began to be released in December, 1941. A total of 1,067 tanks with the Pz.Kpfw.III Ausf.J modification and KwK39 L/60 cannon were produced.